The chairman of the Senate Homeland Security Committee has requested documents to help determine what the Obama administration knew about Russia’s involvement in the 2016 general election prior to President Trump defeating Democratic nominee Hillary Clinton.

Sen. Ron Johnson, Wisconsin Republican, made the request in a letter sent to Homeland Security Secretary Kirstjen Nielsen on Wednesday seeking documents related to a September 2016 briefing in which lawmakers learned from top Obama officials about the Russian government’s attempts to interfere in the race, albeit months before the administration officially accused Moscow of meddling in the election.

“The briefers- then-Secretary Jeh Johnson, then-FBI Director James Comey and President Obama’s Homeland Security Advisor Lisa Monaco-assured Members that the Administration had the matter under control and asked for Congress’s help in reinforcing public confidence in the election,” Mr. Johnson recalled in his letter to Ms. Nielsen.

Under control or not, U.S. intelligence officials revealed four months later in January 2017 that the Russian government allegedly mounted a multi-pronged attack against the election and Mrs. Clinton’s campaign, in turn triggering several, ongoing federal investigations launched to uncover the extent of Moscow’s activities and any possibles ties to the Trump campaign.

Nearly two years since the initial briefing, Mr. Johnson asked Ms. Nielsen for documents to “understand the threat as it existed at the time,” according to his letter.

Specifically, the senator is seeking documents created or used in preparing Mr. Johnson and DHS for the briefing, as well as communications between DHS, FBI and National Security Council officials and employees referring or relating to the matter.

DHS refrains from commenting on congressional correspondence as a matter of public policy, a representative for the agency told The Washington Times. Mr. Johnson’s letter requests a response from the agency by June 20.

Russian state-sponsored hackers, propagandists, professional trolls and other operatives carried out a Kremlin-authorized attack meant to muddy the 2016 race and particularly Mrs. Clinton’s presidential campaign, U.S. intelligence officials concluded in January 2017. DHS subsequently revealed months later that election systems in several states were targeted by Russian hackers during the same span.

Moscow has denied meddling in the race, and the White House has denied colluding with the Kremlin.